The present invention relates to the field of optical fiber transmission, and more specifically it relates to compensating chromatic dispersion and chromatic dispersion slope in optical fiber transmission systems.
The refractive index profile of an optical fiber is generally described in terms of the appearance of a graph plotting the refracting index to the fiber as a function of radius. The distance r from the center of the fiber is conventionally plotted along the abscissa, and the difference in refractive index relative to that of the cladding of the fiber is plotted up the ordinate. The term xe2x80x9cstepxe2x80x9d, xe2x80x9ctrapeziumxe2x80x9d, and xe2x80x9ctrianglexe2x80x9d are therefore used with respect to index profiles whose graphs are respectively step-shaped, trapezium-shaped, and triangular. These curves are generally representative of the ideal or theoretical profile for the fiber, and fiber manufacturing constraints can yield a profile that departs perceptibly therefrom.
In new high bit rate transmission networks that are in wavelength division multiplex (WDM), it is advantageous to manage chromatic dispersion, in particular for bit rates faster than or equal to 10 gibabits per second (Gbit/s); the idea that for all wavelength values of the multiplex chromatic compensation should accumulate to substantially zero over the link as a whole, so as to limit the extent to which pulses widen. Over an entire transmission system, it is acceptable for the cumulative value of dispersion to be a few hundreds of picoseconds per nanometer (ps/nm). It is also beneficial to avoid zero values for chromatic dispersion in the vicinity of the wavelengths actually used in the system since that makes them more subject to non-linear effects. Finally, it is also beneficial to limit the chromatic dispersion slope over the wavelength range of the multiplex so as to avoid or limit distortion between the channels of the multiplex. This problem of compensating chromatic dispersion and chromatic dispersion slope is particularly severe with very high bit rate transmission systems, typically for WDM transmission systems having per channel rates of 40 Gbit/s and above. The problem becomes more severe with bandwidth increasing up to values greater than or equal to 30 nanometers (nm) or even to 35 nm.
Conventionally, the line fibers used in optical fiber transmission systems are step-index fibers; these fibers are commonly referred to as single-mode fibers (SMFs) and they are described in Recommendation ITU-T G.652. Thus, the Applicant markets a step index monomode fiber under the reference ASMF 200 which presents a chromatic dispersion nulling wavelength xcex0 in the range 1300 nm to 1320 nm, and chromatic dispersion of 3.5 picoseconds per nanometer kilometer (ps/(nm.km)) in a range of 1285 nm to 1330 nm, and of 18 ps/(nm.km) at 1550 nm. The chromatic dispersion slope at 1550 nm is about 0.05 picoseconds per square nanometer-kilometer (ps/(nm2.km)). In conventional transmission systems, that fiber is used for conveying signals at wavelengths close to 1550 nm (band C).
Dispersion-shifted fibers (DSF) have also appeared on the market. These fibers are such that at the transmission wavelength at which they are used, which is generally different from the wavelength of 1.3 micrometers (xcexcm) at which the dispersion of silica is substantially zero, their chromatic dispersion is substantially zero. In other words the non-zero chromatic dispersion of silica is compensated, hence the term xe2x80x9cshiftedxe2x80x9d, by increasing the index difference xcex94n between the core of the fiber and the cladding. The index difference enables the wavelength at which chromatic dispersion is zero to be shifted; it is obtained by introducing dopants into the preform, during manufacture thereof, e.g. by the conventional modified chemical vapor deposition (MCVD) process, which is not described in greater detail herein.
The term xe2x80x9cnon-zero dispersion-shifted fibersxe2x80x9d (NZ-DSF) is used for dispersion-shifted fibers that present positive, non-zero chromatic dispersion for the wavelengths at which they are used. Such fibers, at these wavelengths, present little chromatic dispersion, typically less than 10 ps/(ns.km) at 1550 nm, and they present chromatic dispersion slope in the range 0.04 ps/(nm2.km) to 0.1 ps/(nm2.km). Thus, FR-A-2 790 107 proposes a line fiber which is particularly adapted to transmitting a WDM with channels spaced apart by 100 gigahertz (GHz) or less for bit rates per channel of 10 Gbit/s or more; that fiber has, at a wavelength of 1550 nm, an effective sectional area greater than or equal to 60 xcexcm2, chromatic dispersion lying in the range 6 ps/(nm.km) to 10 ps/(nm.km), and chromatic dispersion slope of less than 0.07 ps/(nm2.km).
To compensate the chromatic dispersion and the chromatic dispersion slope in SMFs or in NZ-DSFs used as line fiber, it is known to use short lengths of dispersion compensating fiber (DCF). Such a fiber presents chromatic dispersion and chromatic dispersion slope of sign opposite to that of the chromatic dispersion and chromatic dispersion slope of the line fiber. An example applicable to an SMF line fiber is given by L. Grxc3xcner-Nielsen et al. in xe2x80x9cLarge volume manufacturing of dispersion compensating fibersxe2x80x9d, OFC""98 Technical Digest TuD5. Other examples of dispersion-compensating fibers adapted to SMFs are described in EP-A-0 935 146, U.S. Pat. No. 5,568,583, and U.S. Pat. No. 5,361,319.
WO-A-99 13366 proposes a dispersion-compensating fiber for use in compensation modules to compensate the chromatic dispersion and the chromatic dispersion slope of a fiber of the type marketed by Lucent under the trademark xe2x80x9cTrue Wavexe2x80x9d. That fiber presents chromatic dispersion in the range 1.5 ps/(nm.km) to 4 ps/(nm.km) and a chromatic dispersion slope of 0.07 ps/(nm2.km). The dispersion-compensating fiber proposed in one of the embodiments presents chromatic dispersion of xe2x88x9227 ps/(nm.km) and a chromatic dispersion slope of xe2x88x921.25 ps/(nm2.km), for a theoretical cutoff wavelength shorter than 1100 nm.
EP-A-0 674 193 proposes a dispersion-compensating fiber for SMF that presents a chromatic dispersion value lying in the range xe2x88x9285 ps/(nm.km) to xe2x88x9220 ps/(nm.km); the theoretical cutoff wavelength is not specified in that document; computations to determine the properties of that fiber show that the theoretical cutoff wavelength is shorter than 1100 nm.
U.S. Pat. No. 5,838,867 proposes a dispersion-compensating fiber for use in line or in a module to compensate the chromatic dispersion of a shifted dispersion line fiber. The chromatic dispersion at 1550 nm for the fibers described by way of example lies in the range xe2x88x9260 ps/(nm.km) to xe2x88x922 ps/(nm.km); the cutoff wavelength as measured on two meters of fiber is shorter than 1000 nm, and computations on the properties of the fibers show that the theoretical cutoff wavelength is shorter than 1100 nm.
Lucent Technologies markets broad-band dispersion-compensating modules (for band C) which serve to compensate the chromatic dispersion and the chromatic dispersion slope of an SMF. The ratio of chromatic dispersion over chromatic dispersion slope in the fiber used in those modules is about 925 nm at a wavelength of 1550 nm. At 1550 nm, the fiber presents dispersion close to xe2x88x92100 ps/(nm.km), and a theoretical cutoff wavelength shorter than 1800 nm. Lucent Technologies also markets dispersion-compensating modules for band C NZ-DSFs. Those modules compensate only 65% of the chromatic dispersion slope of an NZ-DSF of the xe2x80x9ctrue wave reduced slopexe2x80x9d type (chromatic dispersion lying in the range 1.5 ps/(nm.km) to 4 ps/(nm.km) and a chromatic dispersion slope of about 0.045 ps/(nm2.km)). The typical value for the ratio of chromatic dispersion over chromatic dispersion slope is about 150 nm at a wavelength of 1550 nm. At 1550 nm, the fiber presents dispersion close to xe2x88x92100 ps/(nm.km), and a theoretical cutoff wavelength shorter than 1800 nm.
Craig D. Poole et al., in xe2x80x9cOptical fiber-based dispersion compensation using higher order modes near cutoffxe2x80x9d suggests injecting light into a dispersion-compensating fiber using a mode for which the cutoff wavelength is close to the wavelength used. Since chromatic dispersion is large in the vicinity of the cutoff wavelength, the quantity of dispersion-compensating fiber required is reduced. That solution implies using a mode converter on input to the dispersion-compensating fiber; the mode converter must have good efficiency so that all of the light is in fact conveyed in the desired mode.
U.S. Pat. No. 5,999,679 proposes a dispersion-compensating fiber having a large effective area and a theoretical cutoff wavelength longer than 1900 nm. At 1550 nm, two examples of fibers proposed in that document present negative chromatic dispersion close to xe2x88x92280 ps/(nm.km), and a ratio of chromatic dispersion over chromatic dispersion slope of 116 nm or 227 nm. The effective area of the proposed fibers is 19 xcexcm2 or 22 xcexcm2. The proposed profile is a rectangular profile with a buried trench and a ring. For those two examples, bending losses are very large (about 0.3 decibels (dB) at 1550 nm for a coil of 100 turns wound with a radius of 30 millimeters (mm), and about 600 decibels per meter (dB/m) at 1550 nm for winding with a radius of 10 mm). In addition, it is difficult to control polarization mode dispersion in the fibers of that document.
The problem raised by the fibers of that document lies in the losses due to bending and to polarization mode dispersion.
The invention solves this problem. It provides a fiber which can be used as a line fiber or in a module, for the purpose of compensating chromatic dispersion and chromatic dispersion slope in an SMF or an NZ-DSF. The fiber of the invention presents a ratio of chromatic dispersion over attenuation which is better than that in the prior art.
More precisely, the invention provides an optical fiber presenting a theoretical cutoff wavelength longer than or equal to 1800 nm, chromatic dispersion that is negative and greater than or equal to xe2x88x92150 ps/(nm.km), and a ratio of chromatic dispersion over chromatic dispersion slope lying in the range 30 nm to 500 nm for a wavelength of 1550 nm.
Advantageously, the fiber can present one or more of the following additional characteristics:
bending losses less than 400 dB/m, and preferably less than 100 dB/m, for a wavelength in the range 1530 nm to 1620 nm, when the fiber is wound on a former having a radius of 10 mm;
bending losses less than 0.05 dB, and preferably less than 10xe2x88x923 dB, for a wavelength lying in the range 1530 nm to 1620 nm, for a coil of 100 turns on a former having a radius of 30 mm;
a ratio of chromatic dispersion over attenuation less than or equal to xe2x88x92100 picoseconds per nanometer decibel (ps/(nm.dB)), and preferably less than or equal to xe2x88x92150 ps/(nm.dB), for a wavelength lying in the range 1530 nm to 1620 nm;
for a wavelength of 1550 nm, an effective area greater than or equal to 12 xcexcm2, preferably greater than or equal to 15 xcexcm2, or even 20 xcexcm2;
for a wavelength of 1550 nm, chromatic dispersion less than or equal to xe2x88x9220 ps/(nm.km), preferably less than or equal to xe2x88x9250 ps/(nm.km);
for a wavelength of 1500 nm, sensitivity to microbends less than or equal to 1, and preferably less than or equal to 0.5;
an index difference between the index at any point of the fiber and the index of the cladding that is less than or equal to 30xc3x9710xe2x88x923, and preferably less than or equal to 25xc3x9710xe2x88x923;
polarization mode dispersion less than or equal to 0.5 picoseconds per root kilometer (ps/km1/2);
attenuation less than 1 dB/km; and
a theoretical cutoff wavelength longer than or equal to 1850 nm.
The fiber advantageously presents a rectangle or trapezium index profile together with a depressed trench and a ring. In which case, the profile can be characterized by:
the difference (xcex94n1) between the index of the rectangle or of the trapezium and the index of the cladding lies in the range 16xc3x9710xe2x88x923 to 24xc3x9710xe2x88x923, and the radius (r1) of the portion of the fiber presenting an index greater than that of the cladding lies in the range 1.5 xcexcm and 2.3 xcexcm;
the difference (xcex94n2) between the index of the depressed trench and the index of the cladding lies in the range xe2x88x927.5xc3x9710xe2x88x923 to xe2x88x923.5xc3x9710xe2x88x923, and the outer radius (r2) of the trench lies in the range 4.5 xcexcm to 6.9 xcexcm; and
the difference (xcex94n3) between the index of the ring and the index of the cladding lies in the range 3xc3x9710xe2x88x923 to 16xc3x9710xe2x88x923, preferably in the range 3xc3x9710xe2x88x923 to 14xc3x9710xe2x88x923, and the outer radius (r3) of the ring lies in the range 6.8 xcexcm to 8.5 xcexcm.
It is also possible to use one or more of the following characteristics to qualify the profile:
twice the integral of the product of the radius multiplied by the index between zero radius and the outer radius (r1) of the central portion of the fiber presenting an index greater than that of the cladding lies in the range 40xc3x9710xe2x88x923 xcexcm2 to 100xc3x9710xe2x88x923 xcexcm2, preferably in the range 50xc3x9710xe2x88x923 xcexcm2 to 80xc3x9710xe2x88x923 xcexcm2;
three times the integral of the product of the square of the radius multiplied by the index between zero radius and the outer radius (r1) of the central portion of the fiber presenting an index greater than that of the cladding lies in the range 60xc3x9710xe2x88x923 xcexcm3 to 200xc3x9710xe2x88x923 xcexcm3, preferably in the range 70xc3x9710xe2x88x923 xcexcm3 to 150xc3x9710xe2x88x923 xcexcm3; and
twice the integral of the product of the radius multiplied by the index between the inner radius and the outer radius of the ring lies in the range 140xc3x9710xe2x88x923 xcexcm2 to 350xc3x9710xe2x88x923 xcexcm2, and preferably in the range 160xc3x9710xe2x88x923 xcexcm2 to 310xc3x9710xe2x88x923 xcexcm2.
The invention also proposes a transmission system in which the line fiber comprises a step index monomode fiber or a dispersion-shifted fiber, dispersion-compensated by such a fiber. Under such circumstances, the cumulative chromatic dispersion in each channel in the range 1530 nm to 1610 nm can be less than 100 ps/nm, for example, and preferably less than 50 ps/nm, on average for transmission over a distance of 100 km.
The line fiber can be constituted solely by step index monomode fiber or solely by dispersion-shifted fiber. It is also possible to provide for the line fiber to be constituted both by step index monomode fiber and by dispersion-compensating fiber, or indeed for the line fiber to be constituted both by dispersion-shifted fiber and by dispersion-compensating fiber.
Finally, the invention proposes a dispersion-compensating module comprising an amplifier and a segment of the above-described fiber.